Wireless transmitters have become ubiquitous in today's society. Such transmitters can be found in myriad mobile terminals such as mobile telephones, personal digital assistants, laptops with wireless modems, and the like. Such transmitters usually have a transceiver circuit that prepares a signal for transmission, a power amplifier that boosts the signal to an elevated power level for transmission, and an antenna that acts as the air interface for the transmitter.
Due to the proliferation of these wireless devices and the potential for interference therebetween as well as interference with other electronic devices, standards have been promulgated both by the Federal Communications Commission (FCC) and by the various governing bodies that limit emissions for certain types of wireless devices. For example, in Code Division Multiple Access (CDMA) systems, very stringent rules control the power levels of CDMA devices within a cell. This helps limit cross channel interference to acceptable levels. While CDMA systems have one of the more visible power control algorithms, other standards and protocols may also be concerned with controlling the power of transmitters. As another example, Global System for Mobile Communications (GSM) has particular requirements specified in GSM Recommendations 05.05 (subsections 4.2.2 and 4.5.2, and Annex 2) that control the spectral frequency mask for the ramp up and ramp down of the power amplifier output. The GSM rules are in place to prevent interference between communications occurring in time slots operating at the same frequency.
To control power output by transmitters, most transmitters incorporate some form of feedback loop that measures the power proximate the antenna of the transmitter. Specifically, most conventional transmitters measure the power levels between the power amplifier and the antenna. Then, the gain of the power amplifier is adjusted so that the power amplifier outputs the desired output power based on the measured power levels.
As an alternative to the feedback loop used by many manufacturers, the assignee of the present invention makes the RF 5117 power amplifier which incorporates power sense circuitry into the power amplifier. While this methodology is suitable for many applications, it remains possible that the designers of mobile terminals may have different requirements in which having the power sense circuitry incorporated into the power amplifier is unacceptable.
Prior to the technique used in the RF 5117, most detection circuits used a detector diode and capacitor to form a half-wave rectifier. The output of this detector was then compared to a reference voltage. The comparison generated an error signal that was used to control the gain of the amplifier. This arrangement worked reasonably well, but errors in the difference between the reference voltage and the output of the detector translated to inaccurate power levels being output by the amplifier. Two common sources of error in this difference are temperature variations or variations in the power supply voltage.
Motorola has produced a detector that is reasonably tolerant to changes in temperature and power supply voltages as evidenced by U.S. Pat. No. 5,448,770. However, this device has been made in a silicon bipolar process and is unsuitable for implementation in a Gallium Arsenide (GaAs) process. GaAs provides a higher breakdown voltage, has a lower forward turn-on voltage, lower series resistance, and a high cut-off frequency. This combination of factors makes the use of GaAs for circuit design attractive. Thus, there remains a need for a detector that is resistant to errors that result from changes in temperature, resistant to errors that result from variations in power supply voltages, and is capable of being implemented in GaAs processes.